About the oxide superconductor, because that critical temperature (Tc) exceeds the liquid nitrogen temperature, the applications for the superconducting magnet, the superconducting electric power cable and the electric power device, etc. are expected, and the various studies are advanced vigorously.
In order to apply the oxide superconducting wire to the above mentioned field, it is necessary to produce a long wire which has a high critical current density (Jc) and a high critical current value (Ic). On the other hand, in order to obtain a long tape, it is necessary to form the oxide superconductor onto a metallic tape shaped substrate from viewpoint of strength and flexibility. Also, in order to enable use of the oxide superconductor at a practical level which is equivalent to the metallic superconductor such as Nb3Sn or Nb3Al, the Ic value of about 500 A/cm (at 77K, in self-field) is required.
Further, in the oxide superconductor, because the oxide superconductor changes the superconducting characteristic by their crystal orientation, it is necessary to improve the in-plane orientation. Also for this reason, it is necessary to form the oxide superconducting layer onto the tape-shaped substrate. In this case, in order to improve the critical current density, it is necessary that the c-axis of the crystal axis for the oxide superconductor is oriented perpendicularly to the plane of the substrate and that its a-axis (or b-axis) has in-plane orientation in parallel to the plane of the substrate, and it is necessary to maintain the quantum connectivity of the superconducting state well.
As the process for producing the tape-shaped RE-type oxide superconductor, that is, as the process for producing RE1+XBa2−XCu3Oy type oxide superconductor (here, RE means at least more than one kind of element selected from Y, Nd, Sm, Gd, Eu, Tb, Pr or Ho. Hereinafter called RE-type (123) superconductor), MOD process (Metal Organic Deposition Processes) is known.
This MOD process makes metal organic acid salt decompose thermally. And after coating a solution that organic compound of metallic component solves uniformly, by heating this and then by making this decompose, the thin film is formed onto the substrate. This MOD process is non-vacuum process. Therefore, because the crystal growth of the film is possible at low cost and high speed, there is an advantage that this process is suitable to produce the long tape-shaped oxide superconductor.
In MOD process, when the metal organic acid salt as starting material is decomposed thermally, the carbonate of alkali earth metal (Ba, etc.) is usually formed. However, in the forming of the oxide superconductor by a solid-phase reaction through this carbonate, the high temperature heat-treatment more than 800 degrees C. is required. In addition, when the thick film is formed, because the nucleation for crystal growth occurs from parts except the interface of the substrate, it is difficult to control the crystal growth rate. Consequently, it is difficult to obtain the superconducting film which has superior in-plane orientation.
In MOD process, as the process for producing RE-type (123) superconductor without going through the carbonate, by using organic acid salt containing fluorine (for example, TFA salt: trifluoroacetate) as the starting material, the process for obtaining the superconductor through decomposition of fluoride by heat-treatment in a water vapor atmosphere is conducted vigorously. In MOD process which uses this TFA salt as the starting material, the superconductor is made by reaction between amorphous precursor containing fluorine which is obtained after calcination of the coating film and water vapor. And, because it is possible to control the decomposition rate of the fluoride by vapor partial pressure in heat-treatment, the crystal growth rate of the superconducting film can be controlled. Consequently, it is possible to make the superconducting film having superior in-plane orientation. Further, in this process, it is possible to make the RE-type (123) superconductor grow epitaxially from the substrate surface at comparatively low temperature.
As discussed previously, when the tape-shaped oxide superconductor is produced by MOD process, thickening the film to improve the Ic value is essential for practical application. For accomplishing the forming of the thick film by MOD process when the starting material is TFA salt, there are thoughts of increasing the viscosity of the material solution containing TEA salt and of thickening the coating film. However, when the thickness of the coating film per once becomes thick, because gas yield of HF and CO2 by decomposition in heat-treatment increases, the phenomenon that the coating film is scattered in calcination occurs. Consequently, it is difficult to produce the thick film of the tape-shaped oxide superconductor having high characteristic.
For producing the thick film of the superconductor, it is possible to thicken the calcination film by repeating the coating of the material and the calcination. However, in the above-mentioned heat-treatment method for calcination of conventional technology, because the temperature-increasing rate which affects the decomposition rate of the metal organic acid salt in the heat-treatment for calcination is fast, the decomposition of the metal organic acid salt including TFA salt is insufficient. Consequently, there is a tendency that the solvent or the organic chain remains in the film of the oxide superconductor precursor which is obtained by the calcination. Therefore, by the increased temperature of the subsequent heat-treatment for crystallization, the organic chains of the remaining fluoride and so on decompose rapidly, and therefore cracks or pores are generated in the film.
This tendency becomes remarkable when the film of the oxide superconductor precursor of multilayer structure is formed and when thickening the film by repeating the coating and the heat-treatment for calcination. Consequently, the epitaxial growth is difficult when the superconducting film is formed by crystallizing the obtained thick film of the precursor. And it is difficult to obtain the superconducting film having superior in-plane orientation, and the Jc characteristic reaches the limit. In addition, the Jc characteristic deteriorates remarkably by generation of the cracks.
For solving these problems, by controlling the temperature-increasing rate in the heat-treatment for calcination, and then by making the metal organic acid salt decompose sufficiently, the method to accomplish the thick film with high Jc is known (for example, refer to Patent document No. 1).
Besides, by controlling the heat-treatment temperature for calcination at the time of the heat-treatment of the oxide superconductor precursor formed onto the substrate, and/or, by controlling the vapor partial pressure of introduced gas in the atmosphere of the heat-treatment for crystallization, the process for producing thick-film tape-shaped oxide superconductor having high orientation and high Jc is known (for example, refer to Patent document No. 2).    Patent document No. 1: Japanese Patent Publication No. 2003-300726    Patent document No. 2: Japanese Patent Publication No. 2003-34527